As to materials, mountain bike frames have followed and in some cases lead the fashions and technologies, first steel, followed by alloy and now the main choice is either carbon frames or titanium frames.
Frame designs are also running similarly to road bike frames, originally they were very like road bikes, but smaller and possibly with more laid back angles to the head and seat tubes. Now they also have compact frame designs with slopping top tubes, just like road bike frames, but in this case I think road bike frames are copying mountain bike frames.
The best frames on the market are very expensive, Santa Cruz, Klien and Yeti are probably the most sought after, but you don’t have to spend a fortune to get a good mountain bike frame, Trek, Giant, Specialized, and Cannondale make very good, responsive frames at reasonable prices.